The present invention relates generally to the electrical, electronic and computer arts, and, more particularly, to quantum computing.
Quantum computing involves the use of quantum-mechanical phenomena such as superposition and entanglement to perform computation. A quantum computer is used to perform such computation. There are currently two primary approaches to physically implementing a quantum computer: analog and digital. Analog approaches include quantum simulation, quantum annealing, and adiabatic quantum computation. Digital quantum computers employ quantum logic gates to perform computation. Both analog and digital approaches for implementing a quantum computer use quantum bits, typically referred to as qubits, which represent the basic units of quantum information.
Quantum computing based on superconducting qubits has made substantial progress recently. (See, e.g., J. M. Gambetta et al., “Building Logical Qubits in a Superconducting Quantum Computing System,” npj Quantum Information 3, Article No. 2, January 2017, pp. 1-7). However, the reliance on superconductivity imposes severe constraints on quantum processors due, at least in part, to the need to cool the qubits to very low temperatures (e.g., about 10 millikelvins (mK)), and a magnitude of the superconducting gap limits the operation frequency of quantum processors to the gigahertz (GHz) range.
Another significant challenge for conventional quantum computing systems is controlling the loss of quantum coherence, known as decoherence. Quantum computers require that coherent states be preserved and that decoherence is managed in order to actually perform quantum computation. Decoherence times for current quantum computing systems are typically on the order of a fraction of a millisecond, which significantly limits the number of operations that can be performed in a prescribed period of time. As a result, time-consuming tasks may render some quantum algorithms inoperable, since maintaining the state of the qubits for a long enough duration will eventually corrupt the superpositions.